na motor is good for 600ish at the wheels with a fatter head gasket, if you are trying to go further than 550ish safely, GTE swap....the W-58 i the n/a might be a possible weak point, ....wonder if theres a way to get an R-154 out of a MK3 turbo to fit in an MK4, much more cost effective, if not, you either need a built auto, or gotta bite the bullet and purchase the Getrag V-160 6 speed, and it is NOT cheap